In selecting between aerobic and anaerobic treatment options, cell yield from influent organics are often considered. For a given substrate (carbon source) the cell yield for anaerobic treatment is between 6 - 10x less than aerobic waste treatment. This reduction in solids production is often used to justify the higher costs of building an anaerobic system.
The yields for an example substrate is given below: Notice that a lightly loaded aerobic system has much lower cell yield than highly loaded systems.
High Loading Conditions - no carbon limitations (high BOD)
1 unit substrate carbon —> 0.5 units CO2 carbon + 0.5 units cell carbon
Low Loading Conditions - liming substrate supply (low BOD)
1 unit substrate carbon —> 0.7 units CO2 carbon + 0.3 units cell carbon
1 unit substrate carbon —> 0.95 units (CO2 + CH4) carbon + 0.05 units cell carbon